Glass window structure providing reduction of interference fringes

ABSTRACT

A glass window structure which eliminates Jamin interference fringes includes a frame for supporting a pair of glass brackets in spaced relationship. A first glass bracket is supported in the frame and this bracket is cut from a glass ribbon manufactured in a process wherein molten glass is flowed out upon a molten bath and processed thereon so that the glass ribbon is of substantially uniform thickness. A second glass bracket is also supported in the frame and this second bracket is cut from a glass ribbon manufactured in the same manner as the first bracket. The second bracket is of slightly different thickness than the first glass bracket, and the difference in thickness eliminates the Jamin fringes. Other window structures which eliminate the Jamin fringes are structures in which (1) at least one of the glass brackets has a tapering thickness from one side to the other side, (2) both brackets have similar tapering thicknesses with the thicker edges of the two brackets not being positioned on the same side of the frame, or (3) both brackets have different tapering thicknesses with the thicker edges being positioned in any manner.

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[72] Inventors James E. Harvey Primary Examiner-David Schonberg DearbornHeights; Assistant Examiner-T. H. Kusmer Mn! 1- lrl 8 Allomeys]ohn R.Faulkner and William E. Johnson Lindbelg, Northvllle. I" of, Mich. [2i 1Appl. No. 756,437 l l 'l- 1968 ABSTRACT: A glass window structure whicheliminates i 1 palfamed July .lamin interference fringes includes aframe for supporting a '8" Fwd Cmnpmy pair of glass brackets in spacedrelationship. A first glass Dearbomt Mich bracket is supported in theframe and this bracket is cut from a glass ribbon manufactured in aprocess wherein molten glass is flowed out upon a molten bath andprocessed thereon so that [54] gs zgggy ggmggggg 1 the glass ribbon isof substantially unifonn thickness. A second 3 Cllhns,5 Drawingii glassbracket IS also supported in the frame and this second bracket is cutfrom a glass ribbon manufactured in the same [52] US. Cl. 350/319,manner as the first bracket The second bracket is of slightly 6 3 N350/276 different thickness than the first glass bracket, and the difhl.fe ence in thickness eliminates (he f inge3 other win- 0' tructureswhich eliminate the jam, fringes are trucl 161/45, .3- tures in which (1at least one ofthe glass bracketshas a tapering thickness from one sideto the other side, (2) both [56] CM brackets have similar taperingthicknesses with the thicker UNITED STATES PATENTS edges of the twobrackets not being positioned on the same 2,827,739 3/ 1958 Atkeson /24side of the frame, or (3) both brackets have different tapering2,362,611 11/1944 Brown 350/319 UX thicknesses with the thicker edgesbeing positioned in any 2,750,637 6/1956 Browne l6l/45 manner.

GLASS WINDOW STRUCTURE PROVIDING REDUCTION OF INTERFERENCE FRINGESBACKGROUND OF THE INVENTION In recent years the method of manufacturingflat glass has been improved by introduction of a glass manufacturingprocess in which molten glass is flowed out upon a molten bath ofmaterial and processed thereon so as to form a glass ribbon. Thisprocess is known as the float process of manufacturing glass. The glassribbon produced by this process is particularly characterized in itsuniform thickness across the width thereof and in the superiorfire-polished surface on the two principal surfaces of the ribbon.Because of the superior surface finish on the glass so manufactured, theglazing industry now utilizes this type of glass most extensively inconstructing window structures.

Included in the window structures formed by the superior quality glassmanufactured by the float process are thermal window structures. In such'a structure, a pair of glass brackets are mounted in a spaced,generally parallel relationship in a window frame. The thermal windowstructures may be utilized in any building wherein it is desired to havea window area sewing as the closure element between the exterior and theinterior of the building.

After some experience with the formation of thermal window structuresutilizing the fire-polished, float glass of uniform thickness, personsin the glazing industry noted that fringes of color could be seen in thestructures when the structures were viewed against a dull backgroundunder certain lighting conditions. The fringes of color gave a woodgrain or oil stain appearance in the structure to the viewer's eye.Fringes of a generally similar appearance have in the past been familiarto fabricators of thermal windows. The previously known fringes could bemade to disappear by inclining one glass bracket very slightly relativeto the other. This remedy fails with some fringes especially those oftenassociated with float glass, since the amount of inclination required toeliminate them is so great as to make the glass brackets conspicuouslynonparallel, and moreover to exceed the space available in the frame ofa thermal window structure.

Thus, although the thermal window structures constructed with floatglass had many advantages, because of the superior surfacecharacteristic of such glass, the structures were objectionable becauseunder certain lighting conditions color fringes were visible. Thepurchaser of such a window structure generally felt that the structurewas defective because of the appearance of the fringes. Thus, thepurchaser of the structure was dissatisfied with the quality of thestructure even though superior glass had been utilized in itsfabrication.

SUMMARY OF THE INVENTION This invention relates to a glass windowstructure and, more particularly, to a glass window structure such as athermal window structure which utilizes float glass in its construction.In detail, the invention is directed to a window structure whicheliminates the objectionable fringe pattern produced in such a structurewhen float glass of uniform thickness is utilized therein.

A glass window structure in accordance with an embodiment of thisinvention employs a window frame for supporting a pair of window glassbrackets in spaced, generally parallel relationship. A first glassbracket supported in the frame is cut from a glass ribbon manufacturedby the float process, that is, the process of manufacturing glasswherein molten glass is flowed out upon a molten bath and processedthereon so that the glass ribbon produced thereby is of a substantiallyuniform thickness, i.e. of a thickness substantially more uniform thaneither plate glass or sheet glass. A second glass bracket is supportedin the window frame and the glass utilized for this bracket is alsomanufactured by thefloat process However, the second glass bracket has auniform thickness which is slightly different than the uniform thicknessof the first glass bracket. The thickness difference between the twoglass brackets is sufficient that when a beam of light is incident upona surface of one of the brackets, the path length for two beams of lightgenerated from the incident light by internal reflections in each of thebrackets are sufficiently different in length that the generated, beamsdo not eventually recombine to produce an interference fringe.

. Thus, in one embodiment of the window structure of this invention, theobjectionable fringe pattern obtained by using float glass of uniformthickness is eliminated by using varying thicknesses of float glass forthe two window brackets, the thicknesses of the two brackets differingsufficiently that certain conditions are met whereby beams of lightgenerated by an incident ray cannot come back together to producefringes.

In alternate window structures, in accordance with additional teachingsof this invention, at least one of the glass brackets of the windowstructure is formed of a glass ribbon manufactured in a float processwhich produces glass having a taper across the width thereof. When thistapered glass bracket is assembled with a glass bracket of uniformthickness, the objectionable fringes produced by two uniformly thickbrackets of float glass are eliminated. Similarly, two brackets oftapered float glass may be utilized in the window construction toeliminate the fringe pattern. However, in this case, if the amount oftaper is the same for both brackets, the thicker edges of each bracketmust not be located on the same side of the window frame and,preferably, are on opposed sides of the window frame;

The window structure, in accordance with the teachings of thisinvention, is one which eliminates the interference fringes observedwhen two brackets of uniformly thick float glass are utilized in theconstruction of a two bracket window structure. The window structures,in accordance with this invention, do not have the particularinterference fringe problem and they are, therefore, more popular withthe glaziers and their customers because the customers do not complainabout the fringe problem.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an isometric view showing anillustration of the occurrence of interference fringes hereinaftercalled .lamin fringes between a spaced pair of parallel glass bracketswhich have uniform and equal thicknesses.

FIG. 2 is an illustration of the manner in which the .lamin fringes aredeveloped, the illustration not taking into account,

the angles of incidence and refraction and secondary rays of light.

FIG. 3 is an illustration of a window structure formed in acv cordancewith the teachings of this invention.

FIG. 4 is an illustration of a second window structure formed inaccordance with the teachings of this invention.

FIG. 5 is an illustration of still another window structureformed inaccordance with the teachings of this invention.

flowed out upon a molten bath and while in contact with the bath, theglass is cooled to a temperature whereat the glass is self-supporting.During the cooling of the glass, the glass will assume a conditionwherein the thickness of the glass across the width of the glass ribbonis substantially uniform and equal. The process of manufacturing floatglass nearly always includes a stretching operation while the glass isplastic and in contact with the float bath. This stretching operationreduces glass thickness below that which occurs naturally as a result oftion when viewing an object therethrough. One particular type of windowstructure which is extremely popular is the thermal window wherein apair of spaced and substantially parallel glass brackets are mounted ina window frame. Such a window structure may be utilized as a closureelement for a building,

and there is generally no need to place a storm window in front of thewindow structure during cold weather.

In past practice, when thermal window structures have been constructedby utilizing glass manufactured by the float process, a window structurehaving superior surface characteristics was achieved. However, certainunanticipated difficulties with the window structures constructed withfloat glass were sometimes encountered. More particularly, when lightincident upon the window was viewed under particular conditions, theviewers eye observed many iridescent marks in the window structure whichcould characteristically be described as a wood grain or oil stainappearance.

The iridescent marks in the thermal window structure formed by twopieces of uniformly thick float glass can best be understood byreference to FIGS. 1 and 2 of the associated drawings. ln P16. 1 athermal window structure, generally designated by the numeral is shownas being formed of two glass brackets 11a and llb. Each bracket is ofuniform thickness and is substantially identical in thickness to theother bracket. This uniformity in thickness would occur in the normalconstruction of such a window structure because adjacent, or almostadjacent, glass brackets cut from a continuous ribbon of glass would beutilized to construct the window structure. Adjacent brackets would beutilized because of the manner in which glass manufacturers package andship the brackets to glass fabricators. The uniformity in thickness andthe continued uniformity in thickness between the brackets of glass cutfrom a glass ribbon is a characteristic of the float process. ln H6. 1 aplurality of spacing blocks 12, representative of a window frame, areshown for holding the glass brackets 11a and llb in spaced,substantially parallel relationship.

When white light is projected onto the upper surface ofwindow structure10 and the structure is mounted against a dull background, iridescentpatterns of fringes 13 are visible in the structure 10. The fringes 13,as schematically represented in the drawing of FIG. 1, cover the fullspectrum of light. While the fringes are not vivid, the colors areobservable and are, therefore, irritating to a person who has paid forthe structure. Three varieties ofwhite light are defined in scientificwork and are known as illuminants A, B and C. These were first definedin an article by D. B. Judd, The [.Cl. Standard Observer and CoordinateSystem for Colorimetry" in the Journal of the Optical Society ofAmerica, volume 23, pp. 359-364.

The fringes observed in the window structure are of the type identifiedas Jamin fringes. The production of Jamin fringes occurs, as any othertype of fringe, when a pair of optical path lengths through the glasswindow structure are substantially identical. The Jamin fringes aredistinguished from the other fringes by the following: I) all foursurfaces of the two glass brackets are involved, and (2) the pathlengths of the beams in each bracket are identical in length. The pathsof the two beams are thus symmetrically disposed relative to one anotherwithin the two brackets. lt is this symmetry which makes the Jaminfringes sensitive to the differences in thickness of the and T2, of thetwo glass brackets 11a and 11b and are quite tolerant to the anglebetween the two glass brackets. Thus, if the two glass brackets are notexactly parallel to one another, the Jamin fringes can still occur'aslong as the two brackets are of equal and uniform thickness. Theinterference fringes are referred to as Jamin fringes because of thesimilarity of their origin to that of the fringes produced by a Jamininterferometer; see Jenkins and White, Fundamentals of Optics, 3rd. Ed.,New York, McGraw Hill lnc., I959, page 257.

With reference to FIG. 2, the .lamin fringes are produced when the beamof light 15 from the light source 1 passes into the first bracket lla tothe point P, at which point a portion of the light beam is reflected togenerate a new beam of light 16 and a portion of the light passes on tothe second bracket llb. Another portion of the beam 15 is reflected atpoint R in the bracket-11b to generate a second new beam of light 17.The new beams of light 16 and 17 are again reflected at points 0 and S,respectively, to generate new beams 18 and 19, respectively, the beam 18being shown as a dotted line.

The thicknesses T1 and T2, respectively of the brackets 11a and 11b, aresubstantially equal within a few light wave lengths of one another andthe angle between the two brackets is approximately l. Under theseconditions, the beam 18 will fall upon the beam 19 and both will bereflected at point V to produce a new beam 20 which is observed by anobserver from point E. lnterference results from the wave properties oflight. When light waves from a single source, or two mutually coherentsources, reach the eye (or other suitable detector) in the same phase,they reinforce; when their phases are opposite, they annul one anotherwholly or partially. lnterference fringes are seen when the phaserelationships of two coherent beams of light alternate in adjacentregions, producing alternately reinforcement and annulment. Each fringeis the geometrical locus of a constant phase relationship between thetwo beams. White light is coherent only for path differences of about0.000] inch. The .lamin fringe occurs, then when the optical path lengthfrom PRS is essentially equal to the optical path length POS whereby thebeams of light come together to form a new beam.

Thus, the highly desirable characteristic of the float glass, namely,its uniform thickness over extended lengths, results in the productionof a thermal window structure which shows interference fringes againstcertain backgrounds and light con ditions. While the interferencefringes are not severe, they are annoying to one who has paid for thestructure. Theaverage customer considers that a window structure whichproduces such interference fringes has not been properly manufactured.

Even though a customer does have a superior glass structure in manyways, he is generally dissatisfied with the structure because of theinterference fringes which are visible.

Window Structure The interference fringes of the Jamin type can beeliminated from the glass window structure by following the teachings ofthis invention. More particularly, the thicknesses of each of the twobrackets of glass utilized to form the window structure should bedifferent. The difference must be large enough that when a beam of lightis incident upon the surface of one of the brackets, the path lengthsfor the beams of light generated therefrom by internal reflections ineach of the brackets are sufficiently different that the generated beamsdo not eventually recombine to produce an interference fringe.

ln the preferred construction, both of the window brackets are formed offloat glass and each bracket is of uniform thickness throughout itsentire extent. There is, however, a difference in. thickness between thetwo brackets of at least 0.0001 inch and, preferably, a difference of atleast 10 wave lengths of visible light. The construction shown in FIG. 3is one wherein a pair of glass brackets 25a and 25b are mounted in awindow frame 26 in spaced and substantially parallel relationship. Theglass bracket 25a is slightly thicker than the glass brackets 25b. Inthis case, when a beam of light 15 from the source I strikes the firstglass bracket 25:: and divides at point P, the subsequently developedbeams 18 and 19 do not recombine with one another to produce aninterference fringe. The

thickness between the two sheets must be at least 0.000l inch and is,preferably, l0 wave lengths of visible light to change the path lengthsof the beams sufficiently that no recombination of the beams takesplace.

ln accordance with a different embodiment of the window structure ofthis invention, as schematically presented in FIGS. 4 and 5, a floatglass ribbon of nonuniform thickness is cut so as to produce glassbrackets varying in thickness from one edge to the other edge. A methodof manufacturing float glass having a tapering thickness from one edgethereof to the other edge thereof is described and claimed in copendingU.S. Pat. application Ser. No. 756,439, filed Aug. 30, l968 and assignedto the same assignee as this application. Briefly, in the methoddescribed in that application, molten glass is allowed to flow out upona molten bath and is processed on the bath to obtain a taper across thewidth thereof by controlling the temperature in certain portions of theglass ribbon while lateral stretching forces are applied thereto. Moltenglass is initially Y flowed out upon the bath and cooled to become asemirigid ribbon of glass. Thereafter, the glass ribbon is reheated butto varying degrees across the width thereof. When the lateral stretchingforces are applied to the glass, the cooler portion of the glass isattenuated or stretched more than the hotter portions of the glassthereby forming a taper across the width of the glass ribbon. The taperformed in the glass ribbon has an angle of about 1 minute of arc. Forfurther details of the process of making tapered float glass, referenceis made to the mentioned application.

With reference to FIG. 4 an alternate embodiment of the window structurein accordance with this invention is shown. In this constructions a pairof glass brackets a and 30 b are mounted in frame members 31. As shownin H6. 4, the taper of the glass bracket 30a is overemphasized. Innormal construction this bracket would taper approximately 0.0l0 inchfor every 100 inches of width. The glass bracket 30b is of uniformthickness across its entire width. In this structure, a beam of light 15from light source l will produce two beams through the structure ofunequal path length. The.resulting rays of light emerging from thestructure generally will not be incident upon one another and will,therefore, not recombine to cause the interference problem except for avery small cen- I tral portion thereof where the fringes areinconspicuous to the unaided eye.

ln FIG. 5 still another construction for the glass window structure isshown. In this case, both the glass brackets 40a and 40b, which aremounted in the frame members 41, are tapered. However, as is apparentfrom FIG. 5, the tapers in the brackets are opposed. In this case, lightincident upon the bracket 40a will divide into two beams which will haveunequal path lengths and, therefore, will not recombine whereby theJamin fringe problem is again overcome except for a narrow band of thestructure where the glass thicknesses are identical. As shown in FIG. 5,the two glass brackets 40a and 40b are positioned so that the thick edgeof the glassbracket 40a is associated with the left frame member 41while the thick edge of the bracket 40b is associated with the rightframe member 41. In construction of the window structure, the onlyrequirement is that the thick edges of the two brackets should not be onthe same side of the window frame because in such a situation theoverlying areas of glass wound have a substantially uniform thicknessand a Jamin fringe pattern could be produced. lf the taper in two glassbrackets is substantially different then the thick edge of both of thebrackets may be positioned on the same side of the window frame.

There has been disclosed in this application a new window structure. Thewindow structure is one which eliminates objectionable fringe patternsin thermal window structures and still permits the utilization in suchstructures of glass manufactured by the float process. The lassmanufactured by the float process has superior surface c aracteristicsand, therefore, is

most desirable in forming the thermal window structure.

The invention disclosed will have many modifications which .will beapparent to those skilled in the art in view of the teachings of thisspecification. It is intended that all modifications which fall withinthe true spirit and scope of this invention be included within the scopeof the appended claims.

What we claim is:

l. A glass window structure comprising:

a frame for supporting a pair of glass brackets in spaced,

generally parallel relationship;

' a first glass bracket supported in the frame, the glass bracket beingcut from a glass ribbon manufactured in a process wherein molten glassif flowed out upon a molten bath and processed thereon so that the glassribbon is of subs antially uniform thickness; and

, a second glass bracket supported in the frame, the second glassbracket being cut from a ribbon of glass manufactured by the sameprocess as the first glass bracket but the second glass bracket being ofa uniform thickness slightly different than the first glass bracket, thethickness of the two glass brackets being sufficiently different thatwhen a ray of light is' incident upon the surface of one of the bracketsthe path lengths for two rays of light generated from the incident rayby internal reflections in each of the brackets are sufficientlydifferent in length that the generated rays do not eventually reinforceon another to produce an interference fringe.

2. The glass window structure of claim 1 wherein the difference inthickness between said two glass brackets is at least 10 wave lengths ofvisible light.

3. The glass window structure of claim 1 wherein the difference inthickness between said two glass brackets exceeds 0.0001 inch.

1. A glass window structure comprising: a frame for supporting a pair ofglass brackets in spaced, generally parallel relationship; a first glassbracket supported in the frame, the glass bracket being cut from a glassribbon manufactured in a process wherein molten glass if flowed out upona molten bath and processed thereon so that the glass ribbon is ofsubstantially uniform thickness; and a second glass bracket supported inthe frame, the second glass bracket being cut from a ribbon of glassmanufactured by the same process as the first glass bracket but thesecond glass bracket being of a uniform thickness slightly differentthan the first glass bracket, the thickness of the two glass bracketsbeing sufficiently different that when a ray of light is incident uponthe surface of one of the brackets the path lengths for two rays oflight generated from the incident ray by internal reflections in each ofthe brackets are sufficiently different in length that the generatedrays do not eventually reinforce on another to produce an interferencefringe.
 2. The glass window structure of claim 1 wherein the differencein thickness between said two glass brackets is at least 10 wave lengthsof visible light.
 3. The glass window structure of claim 1 wherein thedifference in thickness between said two glass brackets exceeds 0.0001inch.